The present invention relates to processes for preparing mechanical pulps having high brightness from wood chips having low bleachability, and more specifically to a pretreatment for extracting causative factors responsible for low bleachability from wood chips having low bleachability.
As for mechanical pulps, the main properties of their quality depend on the nature of the wood fibers from which they are prepared. However, even wood species previously known to be unsuitable for mechanical pulps have recently been used as starting materials because of changes in the demand for application of wood and pulp quality as well as changes in the supply of forest resources relating to the momentum of environmental protection. These wood species used as starting materials often fail to meet desired qualities when they are converted into pulps under conventional process conditions. On the other hand, high value-added papers such as lightweight coated (LWC) paper and supercalendered (SC) paper have recently attracted attention as grades of papers containing mechanical pulps, so that there are demands for a technique for preparing pulps with a quality comparable to or higher than those of conventional pulps from starting materials unsuitable for mechanical pulps.
M. Jackson mentions conifers such as Douglas fir, Jack pine and Larch as starting materials unsuitable for mechanical pulps in 1998 Tappi Pulping Conf. Proc. pp. 455-465. These materials are especially disadvantageous in their low brightness and they require large quantities of bleaching agents such as hydrogen peroxide during the bleaching step to attain a desired brightness because they contain high levels of polyphenolic extractives which consume bleaching agents.
In particular, these species have the disadvantage that the heartwood is colored because it contains high levels of extractives. Mechanical pulps prepared from sapwood alone seem to have qualities closely comparable to those obtained from conventional wood species, but the brightness is lowered when heartwood containing higher levels of extractives than sapwood is included in starting materials and large quantities of bleaching agents have to be added to reach a desired brightness.
Prior techniques for improving the brightness of mechanical pulps are described in several prior applications as follows. JPA SHO 56-85488 discloses a technique comprising pretreating wood chips with 0.5-3.0% by weight of an alkali on the basis of bone dry chips and 0.2-0.7 times the amount of hydrogen peroxide based on the alkali before bleaching them with hydrogen peroxide in a refiner. Japanese Patent No. 1240510 describes a process for preparing bleached mechanical pulp from wood chips, comprising defibrating wood chips in the presence of an organic chelating agent and a sulfite and then bleaching unbleached pulp with a peroxide. Japanese Patent No. 1515223 describes a refiner bleaching technique for preparing bleached mechanical pulp by refining wood chips in the presence of an alkaline hydrogen peroxide bleaching solution, comprising primary refining with an alkaline hydrogen peroxide bleaching solution containing an alkali in an amount enough to attain, after primary refining, pH 9.0-11.0, and then, after primary refining, adding 0.05-3.0% by weight of a mineral acid on the basis of bone dry pulp during the period from the instant immediately after primary refining to the instant immediately before secondary refining, followed by secondary refining. Japanese Patent No. 1515224 describes a refiner bleaching technique for preparing bleached mechanical pulp by refining wood chips in the presence of an alkaline hydrogen peroxide bleaching solution, comprising primary refining with an alkaline hydrogen peroxide bleaching solution containing an alkali in an amount enough to attain pH 7.0-9.0 exclusive after primary refining and then, before secondary refining, adding an alkaline material in an amount equivalent to 5-50% of the amount of the alkali added during primary refining, followed by secondary refining. JPA SHO 59-15589 discloses a process for preparing mechanical refiner wood pulp, comprising a two-stage treatment using sodium sulfite before and after primary refining.
However, none of these prior techniques focused attention on the fact that extractives such as polyphenols contained in conifers are causative factors for lowered brightness, nor did they intend to positively remove these factors to improve the brightness of the resulting bleached mechanical pulp. It would be desirable to develop a novel technique capable of preparing bleached mechanical pulp having high brightness from materials having low bleachability containing high levels of extractives.
The present invention aims firstly to provide a novel technique capable of preparing bleached pulp having high brightness from materials having low bleachability containing high levels of extractives and secondly to provide a technique capable of reducing the amount of bleaching agents used in processes for preparing bleached mechanical pulps.